DE102010021649B4 - Method for producing a glass fiber with an optical fiber core and / or a pump core and such glass fiber - Google Patents

Abstract

A method of producing a glass fiber having an optical fiber core and / or a pump core with fabrication of a preform and a subsequent drawing process, wherein a series of gas reservoirs (10) is provided, wherein a first reservoir (11) with a reducing gas, a second reservoir (12) with an inert gas and a third reservoir (13) for providing an oxidizing gas mixture is provided, and by a distribution unit (8) under the influence of a control unit (7) via opening and closing of the gas reservoirs, a process atmosphere is generated, either pure reducing, inert, both reducing and oxidizing or purely oxidizing, and wherein each set mixture is supplied via a series of leads (16) to the respective process stations, wherein during an execution of manufacturing steps in the preform production in the Glassootabscheidung ( 1), the setting of the degree of sintering (2), the O xidationsschritt (3) and the consolidation (4) takes place with an oxidizing gas atmosphere of chlorine or oxygen with increasing in the consolidation step diffusion length and during the execution of a collapse and a closing step (5, 6) an application of a reducing gas atmosphere and / or during the drawing process is subjected to a reducing or oxidizing gas atmosphere, wherein the gas atmosphere has a variable composition, wherein the variable composition of the gas atmosphere, depending on the respective production step of the Glassootschicht (1), the setting of a defined degree of sintering (2), the Oxidation step (3), a consolidation step (4), the collapse (5) and the pulling of the fiber (6), the case present processing state of a glass material and / or thereby existing diffusion lengths of vorgeseh for the gas atmosphere a gas component (11, 12, 13, 14, 15) in the deposited soot layer, preform or drawn fiber, wherein diffusion of reactive gases into the glass material to affect the internal state of the glass material.

Description

The invention relates to a method for producing a glass fiber with an optical fiber core and / or a pump core according to claim 1 and a glass fiber according to claim 8.

Such glass fibers are manufactured by drawing a preform in a drawing furnace arrangement, wherein the preform is produced in a multi-stage process. In general, for this purpose, first a glassoot layer is deposited on the inside of a substrate tube, which is sintered in a subsequent step. In order to produce a preform structure of a fiber core and a fiber cladding, an oxidation step and a consolidation step are subsequently carried out, followed by collapse and closure.

The publication JP S56-017937 A describes the preparation of a base material for an optical fiber. There, a process step of a so-called axial vapor deposition is described.

The publication US 5,735,928 A discloses an apparatus for making a fused silica article. This document teaches a linear burner assembly for carrying out a vapor phase reaction with a siliceous starting material. The subject of the teaching contained therein is a burner construction in which the burner flame can be modified as such with various admixtures and precursors.

The patent US Pat. No. 6,430,343 B1 describes a beam splitter for use with an optical amplifier. There, inter alia, a so-called "cladding pump fiber" is described, in which a core is provided with an obviously circular cross-section, which is surrounded by a so-called first cladding, which has a star-shaped cross-section. According to the teaching of the document, the first cladding is provided as a pumping region, wherein the core itself is unaffected by the shape of the cladding and has a circular cross-section.

The publication US 2005/0 008 313 A1 discloses a fiber structure having a core with a circular cross-section. The core is surrounded according to the teaching of the document by a cladding whose cross-sectional shape is modified.

When using glass fibers with a light guide core and / or a pump core, an effect is observed with increasing operating time, which is referred to as photodarking. This refers to an adverse interaction of the guided inside the fiber core light with structures that are either present in the structure of the fiber or generated by the guided or excited in the fiber light. This effect depends in part on the wavelength of the light coupled into the fiber. It leads to an increasing optical loss in the optical fiber in the optical fiber over time and to a decreasing power density of the generated laser light within the pump core. Photodarking can be observed in particular with Yb-doped glass or laser fibers. In this case, the absorption within the glass material increases at high light intensities, wherein there is an increasing broadband luminescence in the material.

Extensive investigations of photodarking have shown that this effect is due to a not inconsiderable extent by so-called precursors within the preform of the glass fiber. These are defects in the glass structure, with which the light conducted in the fiber interacts and which are amplified by the action of light. These are caused, in particular, by the dopants, such as ytterbium, aluminum, boron or germanium, which are necessary in particular with pump cores, within the silicon oxide matrix.

It is therefore the object of specifying a production method for producing the glass fiber with an optical fiber core and / or a pump core, in which the leading to the Photodarkening precursors can either be eliminated or at least sustainably minimized.

The object is achieved by a method for producing a glass fiber with an optical fiber core and / or a pump core having the features of claim 1, wherein the subclaims contain expedient and / or advantageous embodiments of the production method. With regard to the device aspect, the object is achieved by a glass fiber with an optical fiber core and / or a pump core with the features of claim 8. According to the invention is a method for producing a glass fiber with an optical fiber core and / or a pump core with a production of a preform and a it subsequent drawing process provided, wherein a series of gas reservoirs is provided, wherein a first reservoir with a reducing gas, a second reservoir with an inert gas and a third reservoir for providing an oxidizing gas mixture is provided. By means of a distributor unit, a process atmosphere is generated under the influence of a control unit via opening and closing of the gas reservoirs, which is either purely reducing, inert, both reducing and oxidizing or purely oxidative, and wherein the respectively set mixture is supplied via a series of supply lines to the respective process stations is supplied. Furthermore, according to the present invention, during execution of fabrication steps in preform fabrication in glass boat deposition, sintering degree setting, oxidation step, and consolidation, exposure to an oxidizing gas atmosphere of chlorine or oxygen is made with diffusion length increasing in the consolidation step and collapsing during execution - And a closing step, an exposure to a reducing gas atmosphere and / or during the drawing process, exposure to a reducing or oxidizing gas atmosphere. In this case, the gas atmosphere has a variable composition, wherein the variable composition of the gas atmosphere depending on the respective manufacturing step of the glassoot layer, the setting of a defined degree of sintering, the oxidation step, a consolidation step, the collapse and the drawing of the fiber, the thereby present processing state of a glass material and / or of the diffusion lengths of the gas components provided for the gas atmosphere in the deposited soot layer, the preform or the drawn fiber. In this case, a diffusion of reactive gases into the glass material for influencing the internal state of the glass material takes place.

It is thus assumed that a manufacturing process with a production of a preform and a subsequent drawing process. In this case, during an execution of production steps in preform production and / or during the drawing process, an admission with a gas atmosphere is carried out in order to eliminate and / or reduce the optically activatable precursor states in the material of the preform and / or the glass fiber. The gas atmosphere has a variable composition for this purpose. The variable composition of the gas atmosphere is set in the execution of the method depending on the respective manufacturing step, a currently present state of processing of the glass material and / or thereby present diffusion lengths of gas components provided thereby in the material of the fiber to be produced.

The basic idea of the method according to the invention is thus to influence its internal state by means of an external diffusion of different reactive gases into the glass material such that the precursors present in the glass can be attacked as comprehensively as possible and thus eliminated. Since in the production of the preform or the glass fiber a number of different effects on the glass structure and the precursors produced are of different types, whereby the glass structure for diffusions of different gases is different permeable, according to the invention the acting gas atmosphere in dependence modified each present production step. This ensures that the precursors formed are influenced by the respectively most effective reactive agent, wherein the diffusion within the glass is conveyed through an appropriately adjusted concentration gradient between the environment and the fiber material.

In an expedient embodiment, the composition of the gas atmosphere is changed in such a way that the gas atmosphere alternates between a reducing and an oxidizing composition during the execution of the production steps. Thereby, precursor states in the glass structure can be eliminated either by reduction or by oxidation with the applied gas and within the glass structure.

As a reducing component of the gas atmosphere, in an expedient embodiment, hydrogen is provided with a variable partial pressure. As the oxidizing component of the gas atmosphere, chlorine and / or oxygen and / or ozone is expediently provided with a respective variable partial pressure. Both hydrogen and oxygen / ozone and chlorine are known as effective reducing or oxidizing agents.

Furthermore, it is provided in an expedient embodiment that the gas atmosphere has a chemically inert component, in particular a noble gas. This makes it possible to keep the total pressure of the gas atmosphere constant even with variable partial pressures of the chemically reactive gas components.

In addition to this, in the production of the glass fiber, it is possible to produce a glass fiber core with at least one flank section deviating from a circular shape. In an expedient embodiment, an edge with a wave-shaped course is introduced for this purpose. This allows the in influence the fiber propagating or generated by pumping operations light modes so that a number of the photodarking conveyed optical fiber modes are attenuated and thus the excitation of remaining in the glass material precursors is difficult.

The wave-shaped edge profile is introduced as a W-shape in an expedient embodiment of the method. Such a form has proven to be particularly useful in terms of the mentioned mode stimulation and mode suppression.

A corresponding glass fiber has at least one flank section deviating from the circular shape. This may be wavy and in particular formed in a W-shape.

The manufacturing method according to the invention and the glass fiber according to the invention will be explained in more detail below with reference to exemplary embodiments. To clarify serve the 1 to 11 , The same reference numbers are used for identical or equivalent parts and method steps.

• 1 einen beispielhaften Verfahrensablauf mit grundlegenden Verfahrenskomponenten anhand eines Blockschaltbildes,
• 2 einen zeitlichen Verlauf des Photodarkening-Verlustes in einer optischen Faser bei einer Lichtwellenlänge von 633 nm bei einer konventionell gefertigen und bei einer erfindungsgemäß hergestellten Faser,
• 3 einen zeitlichen Verlauf des Photodarkening-Verlustes in einer weiteren vergleichenden Darstellung,
• 4 einen beispielhaften zweiseitig abgeflachten Pumpfaserkern in einer Doppel-D-Form,
• 5 einen beispielhaften zweiseitig von der Kreisform abweichenden Pumpfaserkern mit einer Doppel-W-Ausführung,
• 6 einen berechneten Leistungsabfall in Abhängigkeit von der Faserlänge für ausgewählte Pumpkern-Ausführungen,
• 7 beispielhafte Darstellungen der Faserlaser-Ausgangsleistung bei einem Pumpfaserkern mit einer Kreisform im Vergleich mit einer Doppel-D-Ausführung und mit einer Pumpfaser in einer Doppel-W-Ausführung,
• 8 eine beispielhafte Ausführungsform eines Pumpfaserkerns in einer vierfachen W-Ausführung,
• 9 eine beispielhafte Ausführungsform eines Pumpfaserkerns in einer achtfachen W-Ausführung,

Show it:

• 1 an exemplary process sequence with basic process components based on a block diagram,
• 2 a time course of the photodarking loss in an optical fiber at a light wavelength of 633 nm in a conventionally fabricated and in a fiber produced according to the invention,
• 3 a time course of the photodarking loss in a further comparative representation,
• 4 an exemplary double-sided flattened spun core in a double D-shape,
• 5 an exemplary double-sided deviating from the circular shape Pumpfaserkern with a double W version,
• 6 a calculated power loss as a function of the fiber length for selected pump core designs,
• 7 exemplary representations of the fiber laser output power in a spatter core with a circular shape in comparison with a double-D design and with a pump fiber in a double-W design,
• 8th an exemplary embodiment of a pump core in a fourfold W-type,
• 9 an exemplary embodiment of a pump core in an eightfold W-type,

1 shows a block diagram of an exemplary process flow. The manufacturing method in the present example includes deposition of a glassoot layer 1 , a setting of a defined degree of sintering 2 , an oxidation step 3 , a consolidation step 4 , a collapse of the inner coated tube 5 and pulling the fiber 6 , The process steps are carried out in the devices provided for this, in particular separation devices, sintering plants, drawing devices and furnaces.

The individual process steps are performed by a control unit 7 supervised. This is with a distribution unit 8th coupled. The distribution unit is connected to a series of connecting lines 9 with available gas reservoirs 10 connected. The gas reservoirs contain a first reservoir 11 with a reducing gas, especially hydrogen, a second reservoir 12 with an inert gas, in particular a noble gas, such as helium, neon or argon, and a third reservoir 13 for providing an oxidizing gas mixture. In the present case, the reservoir contains 13 a reservoir for oxygen 14 and a reservoir for chlorine 15 ,

Depending on requirements, both further reducing gases and other oxidizing gases can be provided.

Depending on the currently executed process step, the distribution unit generates 8th under the influence of the control unit 7 by opening and closing the gas reservoirs a process atmosphere, either purely reducing, inert, both reducing and oxidizing or purely oxidizing and leads each set mixture through a series of leads 16 to the respective process stations 1 to 6 ,

The exact composition of the gas atmosphere is adjusted on the one hand with regard to the expected diffusion lengths of the gases in a glass frit, the preform or the drawn fiber and on the other hand with regard to the precursors to be influenced and produced in the respective steps in the glass material.

An exemplary list of the diffusion lengths of various gases in silicon oxide as a function of the respective production step is shown in the following table: Diffusion length for gas species in SiO ₂ Diffusion length H ₂ in SiO ₂ (μm) Diffusion length He in SiO ₂ (μm) Diffusion length Cl ₂ in SiO ₂ (μm) Diffusion length O ₂ in SiO ₂ (μm) Characteristic. Layer thicknesses (μm) process step Sootabscheidung 742 969 4 71 (1040) Sintering degree setting 590 857 1 39 (156) oxidation 482 700 1 32 (156) consolidation 595 721 6 73 31 Collapse 1549 1788 24 225 300 Shut down 2682 3096 42 389 650

This shows, for example, that the diffusion length for hydrogen as a preferred reducing agent and helium as a suitable inert gas during the entire course of the process are relatively large, but during the sintering degree, the oxidation and the consolidation in each case passes through a minimum.

The diffusion length of chlorine as a possible oxidant is always small in comparison. It essentially only increases during the process step of the consolidation, the collapse and the closing of the glass fiber and only reaches its greatest value during the last-mentioned process step.

A similar course shows the diffusion length for oxygen in silicon oxide as another possible oxidizing agent. Oxygen, in comparison to chlorine, has a greater diffusion length with a pronounced minimum during the process steps of sintering grade adjustment and oxidation. The diffusion length for oxygen only reaches its maximum value here during the collapse and closing of the preform.

The diffusion lengths are of crucial importance, especially when they are compared with characteristic lengths, which are important in the respective process steps. For example, during deposition, the Glassoot glass material having a thickness or diameter on the order of 1000 μm, i. in the range of one millimeter, while just in this process step, the diffusion length of the oxygen or the chlorine in the glass material are low.

This discrepancy between the characteristic layer thickness of the process step which is large in the soot deposition process and the low diffusion length for oxidants such as oxygen or chlorine on the other hand is of importance because during the soot deposition precursors form in the glass material which, on the one hand, plays a role in the mentioned photodarking, but on the other hand oxidatively influenced and can be eliminated to a certain extent. This applies in particular to so-called oxygen-deficiency centers (ODCs) and non-bridging oxygen hole centers (NBOHCs).

Therefore, in the execution of the method during the process steps of the soot deposition and especially during the Sintergradeinstellung an oxidizing gas atmosphere is set in which there is a large concentration gradient between the oxygen content of the environment of the preform to be produced on the one hand and the oxygen content in the interior of the preform. Thereby, the diffusion process of the oxygen into the interior of the soot layer can be effectively driven and the formation of the mentioned oxygen-hole centers counteracted.

The setting of an oxidizing gas atmosphere is particularly favorable if the diffusion lengths of the oxygen on the one hand coincide with the characteristic layer thicknesses of the respective process step on the other hand do not differ very strongly. This is especially the case during the steps of sintering grade adjustment and oxidation. A thorough oxidation of the mentioned oxygen defects is no longer possible after the layer consolidation because of the already solidified glass structure. Therefore, the oxidizing gas atmosphere is set as early as possible in the process step of soot deposition.

This is particularly important if precursors are to be influenced, which are caused by dopants and codotands in the glass material, in particular ytterbium, aluminum, boron, fluorine, germanium or phosphorus. For example, aluminum causes a so-called aluminum-oxygen hole center (Al-OHC), which contributes to the effect of photodarking and can be influenced by the action of a reducing agent, in particular hydrogen.

Therefore, the gas atmosphere is now brought from an oxidizing to a reducing state during the process steps of collapse and closure. In contrast to the oxidizing gas atmosphere, which may contain a relatively high proportion of oxygen or ozone, the reducing gas atmosphere is generated in the form of an inert gas with a reducing admixture. In the present example, this means that, for example, an atmosphere is generated which consists of 70% helium and 30% hydrogen, wherein the proportion of hydrogen can tend to be set lower because of the very large diffusion length in this process step. As a result, the diffusion-driving gradient between the concentration of hydrogen in the vicinity of the glass fiber or the preform and the hydrogen concentration in the interior of the glass fiber is kept low and avoids overly radical reduction with the production of new precursors.

2 shows exemplary diagrams of a course of a photodarkening loss in dB / m at a light wavelength of 633 nm as a function of time. In the diagram arranged on the left, the time-dependent photodarking of a glass fiber produced from a conventionally produced preform is shown. In comparison, the right-hand diagram shows the time-dependent photodarking of a glass fiber for whose preform the ambient atmosphere has been changed according to the previously described regime. As the diagrams show, the losses due to photodarking in the case of the conventionally manufactured glass fiber already assume a value of 450 dB / m after a service life of approx. 5 hours. In the case of the glass fiber produced according to the invention, the line losses resulting from the photodarking, even after a period of use of 50 hours, only reach less than one tenth of the loss values of the conventionally manufactured fiber.

3 shows in a diagram a further exemplary comparison between a conventionally manufactured glass fiber and a fiber produced by the inventive method. The diagram shows the losses recorded in the fiber in dB / m as a function of time. The values of the curve A represent the conduction losses of a conventionally manufactured glass fiber, the values of the curve B show the conduction losses of a fiber manufactured according to the invention for comparison. While the line loss as a result of photodarking in the conventional glass fiber has already risen to a value of about 620 dB / m after about 20 hours, the line losses in the fiber produced according to the invention only reach a value of even after an operating time of about 47 hours 0.5 dB / m.

This advantageous behavior of the fiber can be supplemented by a corresponding design of the fiber core, in particular of the pump core. It is assumed that a core shape whose cross-section as shown 4 deviates from a circular shape.

At an in 4 in a first example on the basis of a polar plot shown non-circular pump core cross-section of this is formed in a so-called double-D shape. The cross section has in this design two opposite flat flats 17 on. In between are sections 17a the circular basic shape of the pump core.

5 shows a second embodiment of a core cross section according to the invention. In the present embodiment, wave-shaped portions are opposed to each other in the most general case 18 provided in the contour of the fiber core, which may be W-shaped. In this W-shaped design within the section is in each case an outwardly directed bulge 19 provided in the middle of each of a left and right next dent 20 is limited.

With such a core shape, an effective mode mixture in the glass fiber is achieved, especially in pump cores. The mold can be produced quickly, easily and thus inexpensively, whereby a high degree of flexibility in the fiber design is possible and the disadvantages of many solutions already known from the prior art in applications in the laser fiber high-performance range are avoided.

6 shows an exemplary comparison of an expected power drop of the pump light as a function of the length of a fiber at selected cross-sectional shapes of the pump core. The graph shows the normalized power of non-core absorbed light as a function of fiber length. This decreases substantially exponentially over the fiber length. The power drop for a circular shape of the pump core is represented by curve A in the diagram. In comparison, the practically superimposed curves B and C show the power loss of a fiber with a non-circular pump core cross-section according to the 4 and 5 , It shows that the power absorbed in the core at a circular pump core design at the end of an approximately 5 m long fiber only about half of the power is that at a deviating from the circular shape of the pump core in the form of a double-D 4 or a double-W after 5 is to be expected.

7 FIG. 12 shows the long-term power of a fiber laser as a function of the absorbed pump power for a pump core with a double-D shape and a pump core with a double-W shape. In both configurations of the pump core, a linear dependence between the output power of the laser and the absorbed pump power is achieved. The increase in the two function lines is a measure of the laser efficiency. To identify the symmetry of the pump core design, a symmetry parameter was used. It describes the ratio of the inner circle diameter to the circumference diameter of the pump core cross section. In the present case, the symmetry parameter for the double-D shape is for example 0.89 and for the double-W-shape 0.9.

The 8th and 9 show further possible embodiments for the contour of the fiber pump core of laser fibers.

In 8th For this purpose, a design in the form of a quadruple W core is exemplified. In this embodiment, four W-shaped sections 18 provided, which are distributed uniformly over the circumference of the core and thus in pairs opposite each other and are mirror images of each other.

In 9 an exemplary embodiment of an eightfold W-core is shown. The individual W-shaped sections 18 In this case, as in the present exemplary embodiment, they can merge into one another, but also be partitioned from one another, depending on the ratio between the diameter of the fiber core and the lateral extent of the W-shaped sections. In such a case, circular arc-shaped sections and W-shaped sections are distributed over the circumference of the fiber core in sections.

The inventive method and the glass fiber according to the invention were described by means of embodiments. In the context of expert action, further embodiments are possible. These arise in particular from the dependent claims.

LIST OF REFERENCE NUMBERS

1

Glassootabscheidung

2

Setting the degree of sintering

3

oxidation step

4

consolidation

5

Collapse

6

Shut down

7

control unit

8th

distribution unit

9

interconnectors

10

gas reservoir

11

reducing gas reservoir

12

inert gas reservoir

13

oxidizing gas reservoir

14

Reservoir Oxygen

15

Reservoir chlorine

16

leads

17

flat flattening

17a

circular section

18

wavy section

19

bulge

20

dent

Claims (9)

A process for producing a glass fiber with an optical fiber core and / or a pump core with a production of a preform and a subsequent drawing process, wherein a series of gas reservoirs (10) is provided, wherein a first reservoir (11) is provided with a reducing gas, a second reservoir (12) with an inert gas and a third reservoir (13) for providing an oxidizing gas mixture, and by a distribution unit (8) under the influence of a control unit (7) via opening and closing of the gas reservoir, a process atmosphere is generated which is either purely reducing, inert, both reducing and oxidizing or purely oxidizing, and wherein the respectively set mixture over a series of supply lines (16) is supplied to the respective process stations, wherein during execution of manufacturing steps in the preform fabrication in the Glassootabscheidung (1), the setting of the degree of sintering (2), the oxidation step (3) and the consolidation (4 ) an exposure to an oxidizing gas atmosphere of chlorine or oxygen takes place with an increasing in the consolidation step diffusion length and during the execution of a collapsing and a closing step (5, 6) an exposure to a reducing gas atmosphere and / or during the drawing process is subjected to a reducing or oxidizing gas atmosphere, wherein the gas atmosphere has a variable composition, wherein the variable composition of the gas atmosphere depending on the respective production step of the glassoot layer (1), the setting of a defined degree of sintering (2), the oxidation step (3), a consolidation step (4), the collapse (5) and the drawing of the fiber (6) present Processing state of a glass material and / or thereby existing diffusion lengths of the gas atmosphere provided for the gas components (11, 12, 13, 14, 15) in the deposited soot layer, the preform or the drawn fiber is set, wherein a diffusion of reactive gases into the glass material for influencing the internal state of the glass material takes place. Method according to Claim 1 , characterized in that the variable composition of the gas atmosphere is carried out in such a way that the gas atmosphere during the execution of the manufacturing steps between a reducing and an oxidizing composition changes. Method according to Claim 1 or 2 , characterized in that as a reducing component of the gas atmosphere, hydrogen is provided with a variable partial pressure. Method according to Claim 1 or 2 , characterized in that as the oxidizing component of the gas atmosphere chlorine and / or oxygen / ozone is provided with a respective variable partial pressure. Method according to one of the preceding claims, characterized in that the gas atmosphere comprises a chemically inert component, in particular a noble gas, wherein the total pressure of the gas atmosphere is kept constant at varying partial pressures of the chemically reactive gas components by the chemically inert component. Method according to one of the preceding claims, characterized in that in the manufacture of the glass fiber, a finished manufacturing a Glasfaserpumpkerns with at least one deviating from a circular flank section. Method according to Claim 6 , characterized in that the deviating from the circular shape edge portion is introduced in the form of a wave-shaped edge course in a W-shape. Glass fiber for a light guide and / or an optical resonator, produced by a method according to Claim 1 to 7 characterized by a circular Faserpumpkern with at least one arranged on the circumference of the Faserpumpkerns sections deviating from the circular edge in the form of a flat flat (17) or in the form of a wavy edge (18), wherein between the flat flats (17) or the undulating flanks (18) are portions (17a) of the circular basic shape of the Faserpumpkerns. Glass fiber after Claim 8 , characterized in that the wave-shaped flank is W-shaped.

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